Blast & Intruder Resistant Systems

Thermosash have undertaken a number of projects that have demanded a high level of blast or intruder resistance, working with a leading blast consultant to the New Zealand Defence Force. From the physical testing undertaken we have accumulated some of the best knowledge and expertise available outside the USA in designing, manufacturing and integrating these solutions into our façade systems.

Blast resistant façade systems are most commonly made up of the following elements:

Glass / louvre or infill panel

The frame

The primary structure

A blast resistant window system should not outwardly appear to offer any greater resistance over a standard window system or louvre system. However, the engineering and internal mechanics of these systems offer a substantial upgrade to provide protection to the occupants of the building, or when designed to resist an internal blast, public safety.

Most commercial office building windows form a large portion of the exterior and are the building's weakest point from a blast or intruder prevention protection standpoint. Generally building window systems are designed to provide separation from the inner and exterior environment, providing protection from the elements and safety from falling. The design of window systems is typically governed by the environmental effects of the building's location and windows are designed to accommodate such factors as building sway in terms of highrises in high wind zones, differential expansion and contraction of materials in high temperature range enviroments and high wind pressures on the glazing, to name a few. Glass solutions are based on acceptable deformation allowances (bow - inwardly or outwardly) depending on the positive or negative pressures it is subject to. There comes a time when the pressures are too great and this may be from an extreme weather event, eg a tornado or hurricane, a blast or intruder persistance. This may cause the glass to be subjected to greater than manageable loads and it will go from a relatively plastic form to rupturing as it reaches its design performance limits.

In blast events where the building does not collapse, the majority of injuries occur from flying glass, building elements and general office fittings that turn into projectiles.

The insert panel being glass or louvres is only part of the system. The frame, fixings and primary structure require advanced modelling and at times a 'live test' to confirm the ability to perform as specified.

Often the building targeted by a blast device is not the only one that suffers damage, with neighbouring buildings affected within a large perimeter radius.

Depending on the glass installed; annealed, toughened (tempered) or laminated, the glass will act in different ways should it rupture:

Annealed will form large dagger like shards, which travel at high speeds and may travel deep into the building.

Toughened glass, which is generally 3-5 times stronger than annealed, has a charateristic of forming small cubes when it disintegrates. These are generally a lower danger than annealed but at speed can still cause significant occupant harm in a blast event.

Laminated glass (either annealed or toughened) has the added advantage of the interlayer film which holds the fragments together. Smaller shards or cubes can break away from the laminate, still causing potential harm however.

The addition of specialist films and some advanced performance glass, mitigate some of the above occurances. It should be understood that even a performance glass with films can pose significant occupant danger - as potentially a laminated panel may travel as one large mass.

The glass must be encapsulated in a frame that can withstand such forces, held within an engineered sub-structure.

The importance of early understanding of environmental or man-made pressures that a window system can be exposed to must be considered early on in the design phase.

Blast consideration may not be on every project's list of must haves, however corners of buildings can be subject to wind pressures up to 10 times that to which glass is exposed to on the inner surface of a façade and may still require specific engineering design to withstand similar negative pressures. It is highly recommended to undertake façade wind modelling on large façades to predetermine these pressures early in the procurement process.